Zeolitic materials, both natural and synthetic, have been demonstrated to have catalytic properties for various types of hydrocarbon conversion and chemical processing. It is often advantageous to dealuminate these materials in order to improve their process performance. Performance measures include product selectivity, product quality and catalyst stability.
Conventional techniques for zeolite dealumination include hydrothermal treatment, mineral acid treatment with HCl, HNO.sub.3, and H.sub.2 SO.sub.4, and chemical treatment with SiCl.sub.4 or EDTA. The treatments are limited, in many cases, in the extent of dealumination by the onset of crystal degradation and loss of sorption capacity.
U.S. Pat. No. 3,442,795 to Kerr et al. describes a process for preparing highly siliceous zeolite-type materials from crystalline aluminosilicates by means of a solvolysis, e.g. hydrolysis, followed by a chelation. In this process, the acid form of a zeolite is subjected to hydrolysis, to remove aluminum from the aluminosilicate. The aluminum can then be physically separated from the aluminosilicate by the use of complexing or chelating agents such as ethylenediaminetetraacetic acid or carboxylic acid, to form aluminum complexes that are readily removable from the aluminosilicate. The examples are directed to the use of EDTA to remove alumina.
U.S. Pat. No. 3,551,353 to Chen et al. discloses the dealumination of mordenite by contacting with steam and mineral acid in alternate steps. Hydrochloric acid is the preferred mineral acid.
U.S. Pat. No. 4,052,472 to Givens et al. discloses the use of mordenite catalysts having a silica to alumina ratio greater than 15 for conversion of alkanols. Natural mordenites subject to conventional dealumination may be used. Synthetic mordenites, with or without further dealumination, may also be used in the conversion process.
EP 0 259 526 B1 discloses the use of dealumination in producing ECR-17. The preferred dealumination method involves a combination of steam treatment and acid leaching, or chemical treatments with silicon halides. The acid used is preferably a mineral acid, such as HCl, HNO.sub.3 or H.sub.2 SO.sub.4, but may also be weaker acids such as formic, acetic, citric, oxalic, tartaric acids and the like.
U.S. Pat. No. 4,388,177 discloses modifying the shape selectivity of natural ferrierite by treating with oxalic acid to impart catalytic activity.
Therefore, it is an object of the present invention to provide a process for dealumination of a zeolite having the structure of mordenite to reduce the acidity of the catalyst. It is a further object of the present invention to increase the silica to alumina ratio in a zeolite having the structure of mordenite. It is a further object of the present invention to improve process performance of a zeolite having the structure of mordenite.